Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. Currently LTE-Advanced supports an 8-layer spatial multiplexing mode for eight transmit antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions.
FIG. 1 illustrates a spatial multiplexing operation. As seen, the information carrying symbol vector, s, is multiplied by an NT×r precoder matrix, W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) and hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix W is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the wireless device, the inter-layer interference is reduced.
One example method for a wireless device to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
      max    k    ⁢                                                H            ^                    n                ⁢                  W          k                            F    2  where,                Ĥn is a channel estimate, possibly derived from CSI-RS as described below.        Wk is a hypothesized precoder matrix with index k.        ĤnNk is the hypothesized equivalent channel        
In closed-loop precoding for the LTE downlink, the wireless device transmits, based on channel measurements in the forward link (downlink), recommendations to the eNodeB of a suitable precoder to use. The network node, such as an eNB, configures the wireless device to provide feedback according to the wireless device's transmission mode, and may transmit CSI-RS and configure the wireless device to use measurements of CSI-RS to feedback recommended precoding matrices that the wireless device selects from a codebook. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feedback a frequency-selective precoding report, e.g. several precoders, one per sub-band. This is an example of the more general case of channel state information (CSI) feedback, which also encompasses feeding back other information than recommended precoders to assist the eNodeB in subsequent transmissions to the wireless device. Such other information may include channel quality indicators (CQIs) as well as transmission rank indicator (RI).
Given the CSI feedback from the wireless device, the eNB determines the transmission parameters it wishes to use to transmit to the wireless device, including the precoding matrix, transmission rank, and modulation and coding state (MCS). These transmission parameters may differ from the recommendations the wireless device makes. Therefore a rank indicator and MCS may be signaled in downlink control information (DCI), and the precoding matrix can be signaled in DCI or the eNB can transmit a demodulation reference signal from which the equivalent channel can be measured. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
In LTE Release-10, a new reference symbol sequence was introduced for the intent to estimate downlink channel state information, the CSI-RS. The CSI-RS provides several advantages over basing the CSI feedback on the common reference symbols (CRS) which were used, for that purpose, in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal and, thus, does not require the same density (i.e., the overhead of the CSI-RS is substantially less). Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements (e.g., which CSI-RS resource to measure on can be configured in a wireless device-specific manner).
By measuring a CSI-RS transmitted from the network node, a wireless device can estimate the effective channel the CSI-RS is traversing including the radio propagation channel and antenna gains. In more mathematical rigor this implies that if a known CSI-RS signal x is transmitted, a wireless device can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). As such, if no virtualization is performed in the transmission, the received signal y can be expressed as follows:y=Hx+e The wireless device can estimate the effective channel, H.
Up to eight CSI-RS ports can be configured in LTE Rel-10. Thus, the wireless device can estimate the channel from up to eight transmit antennas.
Related to CSI-RS is the concept of zero-power CSI-RS resources (also known as a muted CSI-RS) that are configured just as regular CSI-RS resources, so that a wireless device knows that the data transmission is mapped around those resources. The intent of the zero-power CSI-RS resources is to enable the network to mute the transmission on the corresponding resources in order to boost the SINR of a corresponding non-zero power CSI-RS, possibly transmitted in a neighbor cell/transmission point. For Rel-11 of LTE, a special zero-power CSI-RS was introduced that a wireless device is mandated to use for measuring interference plus noise. A wireless device can assume that the TPs of interest are not transmitting on the zero-power CSI-RS resource, and the received power can therefore be used as a measure of the interference plus noise.
Based on a specified CSI-RS resource and on an interference measurement configuration (e.g. a zero-power CSI-RS resource), the wireless device can estimate the effective channel and noise plus interference, and consequently also determine the rank, precoding matrix, and MCS to recommend to best match the particular channel.
This disclosure may be used with two dimensional antenna arrays and some of the presented embodiments use such antennas. Such antenna arrays may be (partly) described by the number of antenna columns corresponding to the horizontal dimension, Mh, the number of antenna rows corresponding to the vertical dimension, Mv, and the number of dimensions corresponding to different polarizations, Mp. The total number of antennas is thus calculated by:M=MhMvMp It should be pointed out that the concept of an antenna is non-limiting in the sense that it can refer to any virtualization (e.g., linear mapping) of the physical antenna elements. For example, pairs of physical sub-elements could be fed the same signal, and hence share the same virtualized antenna port.
FIG. 2 illustrates an example of a 4×4 array with cross-polarized antenna elements. Precoding may be interpreted as multiplying the signal with different beamforming weights for each antenna prior to transmission. A typical approach is to tailor the precoder to the antenna form factor by taking into account Mh, Mv, and Mp when designing the precoder codebook.
A common approach when designing precoder codebooks tailored for two-dimensional antenna arrays is to combine precoders tailored for a horizontal array and a vertical array respectively by means of a Kronecker product. This means that (at least part of) the precoder can be described as a function of:WH⊗WV where WH is a horizontal precoder taken from a (sub)-codebook XH containing CH codewords and similarly WV is a vertical precoder taken from a (sub)-codebook XV containing CV codewords. The joint codebook denoted as XH⊗XV contains CH·CV codewords. The elements of XH are indexed with kH=0, . . . , CH−1, the elements of XV are indexed with kV=0, . . . , CV−1 and the elements of the joint codebook XH⊗XV are indexed with kHV=CV·kH+kV meaning that kHV=0, . . . , CH·CV−1.
It should be pointed out that a precoder codebook may be defined in several ways. For example, the above-mentioned Kronecker codebook may be interpreted as one codebook indexed with a single PMI, kHV. Alternatively, it may be interpreted as a single codebook indexed with two PMIs, kH and kV. It may also be interpreted as two separate codebooks, indexed with kH and kV, respectively. Further, the Kronecker codebook discussed above may only describe a part of the precoder. Thus, the precoder may be a function of other parameters as well. For example, the precoder may also be a function of another PMI, n. Again, this can be interpreted as three separate codebooks with indices kH and kV and n, respectively, or two separate codebooks with indices kHV=NV·kH+kV and n, respectively. It may also be interpreted as a single joint codebook with a joint PMI. However, these are only examples as to how a codebook may be defined. Any suitable method may be used for defining the codebook.
One example of the codebook structure with three PMIs is when the precoder W has the following form:
  W  =            (                                                                  W                H                            ⊗                              W                V                                                          0                                                0                                                              W                H                            ⊗                              W                V                                                        )        ⁢          W      n      
DFT-based precoder codebooks for WH, WV are commonly used. When interpreted as a single codebook indexed with two PMIs as described above, the codebook can be expressed as a matrix X:
      X    ⁡          (                        k          H                ,                  k          V                    )        =            e              j        ⁢                                  ⁢        2        ⁢        π        ⁢                              l            ⁡                          (                                                k                  H                                +                Δ                            )                                                          Q              H                        ⁢                          N              H                                            ⁢          e              j        ⁢                                  ⁢        2        ⁢        π        ⁢                              m            ⁡                          (                                                k                  V                                +                Δ                            )                                                          Q              V                        ⁢                          N              V                                          Where:                X(kH, kV) is a matrix of NV rows and NH columns associated with a precoder (or ‘beam’) indexed by kH and kV.        NV and NH represent the number of antenna ports in the vertical and horizontal dimensions.        All elements of X (kH, kV) can be mapped to a column of the precoding matrix W, described above.        l, m are horizontal and vertical antenna port indices, respectively.        QH and QV are horizontal and vertical oversampling factors, respectively.        kH=0, . . . , NHQV−1, kV=0, . . . , NVQV−1 are horizontal and vertical beam indices.        Δ can take on value in the interval 0 to 1 so as to “shift” the beam pattern. (As just one example, Δ=0.5 may be a value for creating symmetry of beams with respect to the broadside of an array.)        